This invention relates, in general, to heterojunction bipolar transistors and, more particularly, to heterojunction bipolar transistors formed on silicon substrates.
Silicon junction bipolar transistors are commonly used as discrete devices and as elements of integrated circuits. Conventional junction bipolar transistors comprise a silicon substrate having a base region diffused in the substrate and an emitter region diffused into the base region. Silicon is a mechanically sturdy substrate on which to form devices, and is compatible with batch processing equipment. Additionally, large diameter silicon substrates are available which allow many devices to be fabricated on a single substrate.
Emitter injection efficiency and current gain of the conventional bipolar junction transistor are limited by resistivity of the base and emitter regions. Lower base resistivity degrades emitter injection efficiency and current gain. High emitter resistivity also degrades emitter injection efficiency and current gain. To improve operating speed, however, it is desirable to have low base resistivity and high emitter resistivity. A significant portion of the delay time associated with a conventional junction bipolar transistor is related to the product of base resistance (R.sub.b) and emitter capacitance (C.sub.e). R.sub.b is directly proportional to base resistivity while C.sub.e is inversely proportional to emitter resistivity. A low time constant R.sub.b C.sub.e requires low base resistivity and high emitter resistivity, resulting in poor emitter injection efficiency.
Heterojunction bipolar transistors (HBTs) are well-known in the semiconductor art. HBTs offer advantages over conventional junction bipolar transistors by providing a heteropotential between base and emitter regions. In an NPN transistor, this heteropotential restricts hole flow from base to emitter, thus improving emitter injection efficiency and current gain. The improved emitter injection efficiency allows the use of low resistivity base regions and high resistivity emitter regions to create fast devices without compromising other device parameters. Thus, HBTs can have high current gain while at the same time having a low base resistivity and low emitter base junction capacitance.
HBTs are usually formed from III-V compound semiconductor materials. Compound semiconductor materials in general are quite expensive and are difficult to work with in a manufacturing environment. In particular, III-V compound materials are quite fragile, so that conventional semiconductor handling equipment is difficult to use. In addition, III-V compound semiconductor wafers are significantly smaller than silicon semiconductor wafers due to the added fragility. All of these factors combine to make processing of III-V materials significantly more expensive than silicon processing.
Attempts have been made to form heterojunction bipolar transistors using silicon substrates. To form an HBT, the emitter must have a wider bandgap than the base. The bandgap differential results in a valence band discontinuity at the base-emitter junction. To achieve this bandgap relationship on a silicon substrate requires that a different material, having a different bandgap than silicon, be deposited epitaxially on the silicon substrate. One such silicon HBT uses a germanium-silicon base upon which a silicon emitter is formed. This structure results in a valence band discontinuity of about 0.2 eV. While this valence band discontinuity serves to block some hole flow from base to emitter, it is desirable to have a larger valence band discontinuity.
Recently, techniques have been developed to epitaxially grow gallium phosphide (GaP) on silicon. GaP-Si junctions are heterojunctions with the GaP having a wider bandgap than the Si. A mismatch in lattice constants between GaP and Si made it difficult to grow high quality GaP films which could be used as emitters. In particular, GaP layers exhibited lattice mismatch defects and cracks when GaP layers of even a few thousand angstroms were made larger than a few square mils. Due to the difficulty in growing GaP on silicon and the poor quality of the semiconductor junction thus formed, GaP-Si HBTs have not been commercially feasible.
Accordingly, it is an object of the present invention to provide a heterojunction bipolar transistor having a silicon substrate.
It is another object of the present invention to provide a heterojunction bipolar transistor having a low resistance emitter.
It is a further object of the present invention to provide a heterojunction bipolar transistor (HBT) having improved operating speed.
It is another object of the present invention to provide a heterojunction bipolar transistor having a coherently strained emitter layer.